1. Technical Field
The present disclosure relates to the field of voltage reference generators and, particularly, to the band-gap voltage reference circuits.
2. Description of the Related Art
As it is known, many electrical circuits employ a voltage reference circuit, which should exhibit little dependence on supply and process parameters and a well defined temperature behavior. A known reference generator technique is the band-gap reference which balances a negative temperature coefficient of a pn junction with a positive temperature coefficient of the thermal voltage, Vth=kBT/q, where kB is the Boltzmann's constant and q the electron's charge. Typically, the two terms having opposite temperature behaviors are the voltage base-emitter Vbe of a BJT (bipolar junction transistor) and the difference ΔVbe between two bipolar transistors. The generated voltage Vbg can be expressed as:Vbg=K1Vbe+K2ΔVbe wherein factors K1 and K2 represent ratio of resistors included in the voltage reference circuit, having the same temperature behavior.
It has been observed that many second order effects cause variation of the derivatives of Vbe and ΔVbe. Consequently, the temperature variations of the two terms indicated in the expression above are still linear, but their second order derivatives have a variable temperature behavior. This situation produces a voltage versus temperature curve (volts/° C.) showing a parabolic behavior as the one exemplary depicted in FIG. 8.
Moreover, the statistical dispersion of silicon parameters during the manufacturing process causes a dependence of the temperature which can be different for each manufactured circuit. Therefore, it is necessary to calibrate a voltage reference circuit. In accordance with known techniques, the calibration occurs during a particular manufacturing step or, after the manufacturing process, in a testing step. The calibration consists in modifying both or one of the factors K1 and K2. The Applicants note that this type of calibration increases the costs of the manufacturing process and does not take into account the performance losses occurring during the circuit life.
Document U.S. Pat. No. 7,433,790 describes a circuit provided with a logic block performing a test algorithm to control trimming of a reference value generating circuit and a temperature measurement system.
Document U.S. Pat. No. 5,440,305 discloses an apparatus for calibration of errors in a monolithic reference including a band-gap voltage reference. Moreover, this document describes a calibration operation in which a temperature measuring system and a burn-in oven are employed and a calculation to determine compensation factors is performed.